This invention relates to a steam generator which comprises an evaporator for producing vapor from the feed-water and a super-heater for further heating the vapor produced and which generates steam through heat-exchange between feed-water and alkaline metal. More particularly, this invention relates to a control system for the steam generator for preventing liquid water content from entrance into the super-heater.
A steam generator where heat-exchange is made between alkaline metal and feed-water is employed in a fast breeder reactor, for example. The alkaline metal is used as a coolant carrying the heat produced in the nuclear reactor to exterior. The steam generated by the steam generator drives a turbine generator.
A rough construction of the steam generator is as follows: A closed tank receives at its one end a high temperature alkaline metal fed from the nuclear reactor while feeds from its other end the alkaline metal cooled as a result of heat exchange back to the nuclear reactor. The closed tank is passed through by a plurality of heat transfer tubes each of which permits the feed-water to pass therethrough. The thermal energy of the alkaline metal is transferred to the feed-water through the heat transfer tube thereby producing water vapor.
Liquid sodium is most preferable as the coolant alkaline metal for heat transfer in the nuclear reactor, from its property and economic standpoints. On the other hand, the liquid sodium belonging to the alkaline metal group with a high activity exhibits a remarkably high chemical activity, and reacts violently with water into explosive burning. In the fast breeder reactor, the steam generator is the place having the highest possibility that the alkaline metal comes in direct contact with water. Since the steam generator is located between the nuclear reactor and the turbine generator, if the alkaline metal should come in direct contact with water, not only the steam generator but also other apparatuses including the turbine generator and the nuclear reactor which constitute the whole nuclear power plant might be all damaged. Thus, the steam generator is very important, from a safty point of view. Such decident almost depends on the mechanical soundness of the heat-transfer tubes. The damage of the heat-transfer tubes is due largely to the thermal fatigue when it is subjected to a high temperature, the corrosion by moisture content, or the like. First, the heat transfer tube portions existing in the region where the vapor produced is superheated must be proof against very high temperature and must be strong against fatigue since both the alkaline metal and the steam flowing in such portion are at a very high temperature. Next, the heat transfer tube portions existing in the region (boiling region) where feed-water flows must be proof against water corrosion. Accordingly, in the steam generator in which the superheated portion and the vapor producing portion are integrally provided, the material of the steam generator should withstand both the high temperature and water corrosion. No material satisfying such two requirements has been found. This results in the advent of the so-called separate type steam generator where the evaporator and the super-heater are separately provided, so that factors which affect the damage of the heat-transfer tubes may be eliminated and the optimum materials for the evaporator portion and for the super heater portion may be individually selected. More precisely, for the evaporator, for example, 21/4 C - 1 Mo steel may be used as the material being resistive to the water corrosion. For the super heater, austenitic stainless steel, for example, may be used as the material durable for a high temperature. In this manner, the heat-transfer tubes are remarkably prevented from the damage thereof.
As described above, for the purpose of prevention of the heat-transfer tubes from the damage, the evaporator and the super-heater have been separately provided in the steam generator, and the materials most suitable for the respective portions have been selected. However, it has been found that the austenitic stainless steel which is the material for the super-heater is weak in liquid water content and hence if liquid water is entered into the super-heater, the heat-transfer tubes on the super-heater are adversely affected. In a rated operation of the nuclear power station, the super heat starting point of the water is located in a given position in the evaporator and the vapor produced therein is entered to the super-heater after it is further heated. For this, the entrance of liquid water content into the super-heater may be almost eliminated. However, when the flow rate of alkaline metal is reduced for decreasing the load of the nuclear power station, the super heat starting point of the water shifts to the outlet side of the evaporator with the result that liquid water content possibly enters the super-heater thereby damaging the heat-transfer tubes.